This invention relates to clamps for lifting articles, such as metal plates, and more particularly to such a clamp wherein the opposed jaws have flat surfaces and are moved together to grip the article to be lifted in such a manner that articles lifted will not have their surfaces seriously damaged incident to the lifting by the clamp.
Further, the lifting clamp of the invention relates to clamps having a locking device to retain the clamp jaws in either an open or closed position.
Renfroe U.S. Pat. No. 2,654,630 exemplifies this latter type of clamp, as does Davies U.S. Pat. No. 4,162,804, the latter patent disclosing an auxiliary lock means to latch the primary locking device against inadvertant movement which could release the clamp to its open position.
Basically, the two above-identified patents disclose clamps having a clamp body with a slot to receive an article to be lifted. A jaw is provided on each side of this slot adjacent the outer open end of the slot. The gripping jaw is movable between open and closed positions relative to the other jaw which is usually fixed. The movable jaw is operated by a shackle connected to the movable jaw by a linkage assembly which characteristically provides force multiplying action in transmitting lifting force between the shackle into clamping force applied to the opposed jaws.
The lifting clamp body includes a pair of spaced side plates with a pivotal locking plate mounted on the inside of the side plates, this locking plate being operated by a handle exposed exteriorally of the clamp body. A heavy tension spring connects the locking plate to the linkage assembly which extends between the shackle and the movable jaw. By rotation of the locking handle to a closed position, the spring exerts a heavy closing force on the movable jaw.
Characteristic of the lifting clamps exemplified by the above-mentioned patents is the fact that the movable jaw is pivotally mounted so that it necessarily swings through an arc into gripping engagement with the surface of the article to be lifted. It also is common practice for this movable jaw, and indeed the opposed stationary jaw, to be provided with gripping teeth which will dig into the surface of the lifted article. While such swinging action of the movable jaw and the provision of gripping teeth on the jaw are advantageous in lifting articles made of steel, there are instances where articles made of materials such as bronze, stainless steel or polished smooth surfaces would be unduly marred or gouged by the swing-in action of a pivoted movable jaw carrying gripping teeth on its clamping surface.
There also are drawbacks in prior art lifting clamps like those of the above-mentioned patents in that the spacing between the opposed jaws of the clamp is not adjustable other than incident swinging in movement of the pivotal jaw of the clamp.
Additionally, the manually engagable lock handle on such lifting clamps frequently must be so located that its swing path extends substantially beyond the perimeter of the clamp body. Thus, when the handle must be shifted between its extremes to effect either the jaw opening or closing operations, this handle projection beyond the clamp body perimeter can be obstructed by closely adjacent items in the environment where the clamp is being used.
Again, in referring to the locking devices associated with prior art lifting clamps, these frequently require full swing manual operation of the handle between its extreme positions either to move the lock handle fully to a locked open position or fully to a locked closed position. In contrast, it can be advantageous to permit the lock device handle to be moved to a neutral position, a position which will not urge opening of the lifting clamp but will only permit the jaws to open when the lifting force is relaxed. Thereafter, as long as the clamp is in a vertical position, the weight of the shackle and remaining biasing force of the spring will automatically move the lock handle from the neutral position to its fully locked open position.
Finally, prior so-called non-marring clamps which incorporate a single throw locking device, are generally bulky and unwieldy for their intended lifting applications. They frequently contain numerous and complicated mechanisms and necessarily end up being very expensive.